Preparation of graft copolymers of poly(vinyl chloride) using free radical grafting techniques is inefficient in that only low amounts of the grafting monomers become chemically attached as grafted polymer to the poly(vinyl chloride) backbone. Compositions resulting from these grafting reactions contain large amounts of ungrafted polymers which are usually incompatible with poly(vinyl chloride). Consequently, the materials have poor physical, optical and processing properties. The main reason for the grafting inefficiency is the low reactivity of the poly(vinyl chloride) towards attack by either initiator or polymer free radicals.
It is known in the literature.sup.1,2,3 that much higher grafting efficiencies can be obtained by using a poly(vinyl chloride) backbone that has been partially dehydrochlorinated. Partially dehydrochlorinated poly(vinyl chloride) contains unsaturated sequences (polyenes) which are quite susceptible to attack by free radicals.
Various techniques have been described for preparing partially dehydrochlorinated poly(vinyl chloride).sup.4. Chemical methods [treatment of poly(vinyl chloride) with bases such as potassium hydroxide or ammonia] are normally carried out with the poly(vinyl chloride) dissolved in a solvent such as tetrahydrofuran in order to get uniform dehydrochlorination.sup.1,3,5. Uniform dehydrochlorination can also be achieved by heating a solution of poly(vinyl chloride) in a high boiling solvent such as ethyl benzoate at temperatures above 160.degree. C..sup.6. Solution techniques are very cumbersome, however, since only dilute solutions (5% by weight) can be used and the resultant dehydrochlorinated poly(vinyl chloride) must be recovered from the solvent by evaporation or precipitation.
Base catalyzed dehydrochlorination of solid poly(vinyl chloride) occurs mainly on the particle surface and there is a strong tendency towards gel formation. More uniform dehydrochlorination of solid poly(vinyl chloride) results from a pure thermal process. To obtain uniform dehydrochlorination on dry, solid resins, the resin particles must be heated and circulated with an inert gas as in a fluidized bed.sup.7. Alternatively, fluids such as ethylene glycol, paraffin oil or maleic anhydride have been used as media in which the resin particles can be circulated to provide uniform heat transfer.sup.2,8. Poly(vinyl chloride) has been dehydrochlorinated in dimethylformamide solution containing lithium chloride.sup.9. Poly(vinyl chloride) has been dehydrochlorinated and sulfonated by treatment in sulfuric acid.sup.10. The dehydrochlorinated poly(vinyl chloride), however, must be isolated from these fluids prior to free radical grafting, and the heat transfer fluids have to be purified before being recirculated. Polyvinyl chloride has been treated in aqueous caustic at 180.degree. to 300.degree. C. to obtain a high m.w. polymer composed mostly of carbon, hydrogen and oxygen with traces of nitrogen and chlorine being present.sup.11.